The invention relates generally to polymer-electrolyte membranes. Particularly, the invention relates to polymer-electrolyte membranes suitable for use in electrochemical fuel cells.
Electrochemical fuel cells convert reactants, namely fuel and oxidant streams, to generate electric power and reaction products. Fuel cells typically oxidize fuels to produce electrons and protons, at a catalyst layer disposed on an electrically-conductive material, serving as the anode. The electrons are conducted through the anode, and produce electrical energy. The protons flow from the anode through an electrolytic medium, often supported by a membrane, to a catalyst layer disposed on the cathode, where they are recombined with electrons to reduce an oxidant. In the case of hydrogen fuel cells, molecular hydrogen is oxidized into electrons and protons at the anode. The electrons so produced are transported through the electrically conductive anode, whereas the protons migrate through a polymer-electrolyte membrane (PEM). The PEM comprises a polymeric scaffold supporting a proton-conductive electrolyte (often aqueous) phase that enables conduction of protons (as electrolyte-bound cations) through the PEM, to the cathode where they recombine with electrons to reduce an oxidant, generally oxygen.
To produce a structure capable of performing the above mentioned electrochemical task, an anode and cathode, each containing catalyst particles, are adhered to opposite sides of the PEM to form a layered composite structure. This composite structure is responsible for the electrochemical conversions and directed flow of fuel, byproducts, ions, electrolytes and electrons requisite for the electrochemical functioning of a fuel cell. This layered composite structure is also referred to as a membrane electrode assembly (MEA).
PEMs may generally exhibit phase separated morphologies, wherein an electrolyte phase is dispersed within a continuous polymeric phase that provides mechanical stability. Both electrolyte and polymer phases can impact the permeation of protons through the PEM. On a molecular level, the chemical characteristics of the electrolyte desirable for proton conduction include molecular topology, mobility, and acidity. Increased electrolyte concentration within the membrane may enhance accessibility and improve rates of proton sorption and permeation though the membrane. Maintenance of constant electrolyte concentration may be desirable for obtaining stable proton conductivity, and replenishment of vaporized or leached electrolyte may be continuously performed during operation. On a morphological level, dynamic fluctuations between electrolyte domains may provide conductive pathways through the polymeric continuous phase. Thus, the ability of the polymeric phase to mechanically comply with the anodic proton flux may enable proton percolation though the membrane and enhance conductivity.
It is also desirable that the membrane be mechanically robust during operation, and serve as a dielectric and mechanical barrier to the electrical and electrochemical processes within the MEA, while simultaneously conducting protons. Thus typical polymers for PEMs possess oligomeric regions that are non-interactive with the electrolyte, but rather, are involved in intermolecular associations to provide a mechanically robust, continuous phase. In addition, these polymers have regions compatible with the chosen electrolyte; and these regions generally contain acidic or basic residues. PEMs of particular interest comprise acid-functional polymers such as acid-functional fluoropolymers, and electrolytes, such as, water. Suitable examples of acid-functional fluoropolymers include electrolyte non-interactive (or hydrophobic) oligo(perfluoroethylene) blocks; and electrolyte interactive fluorosulfonic acid residues pendant to the polymer chain.
The ability of the PEM to separate electrochemical processes by selectively restricting permeation of fuel, oxidant and byproducts is also desirable. This process is often referred to as “crossover”. Thus, in order to improve efficiencies, it may be desirable to have a PEM with low fuel crossover.
When water is used as an electrolyte, the conductivity of the PEM may suffer at temperatures greater than 80° C. This is a result of evaporative depletion of the electrolyte phase in the PEM. Humidification of the membrane thus improves performance of the fuel cell over extended periods. However, the cell performance may further degrade at higher temperatures, particularly over 100° C., where there is reduced water absorption. As the vapor pressure of water increases rapidly with temperature, it becomes necessary to operate the fuel cell at higher pressures, which increases system design complexity and cost. Thus, there is a need to operate under low relative humidity conditions, even at normal operating temperatures.
Accordingly, there is a need for PEM configurations that improve electrochemical performance when operated under low humidity and/or high temperature conditions. Further, it may be desirable to have such PEM configurations suitable for use within the fuel cell environment.